142 PHILOSOPHICAL TRANSACTIONS. [aNNO 177/. 



converted into vapour. After 4 hours the saucers were again accurately weighed ; 

 that in the open air had lost 1 drachm and 8 grains ; the weight of the other 

 was not sensibly diminished. From this experiment it appears, that air is a 

 chemical solvent of water, and as such is undoubtedly to be considered as one 

 cause of the eva])oration of water. Heat is another cause of evaporation, and 

 when raised to a sufficient degree may produce this effect without the interven- 

 tion of air, and the evaporation consequently go on copiously in an exhausted 

 receiver, agreeably to the experiments of the ingenious Dr. Irving, in Phipps's 

 Voyage to the North Pole, p. 'ill. 



The following observations are added as a further illustration of this subject. 

 Water may exist in air in 3 different states. 1 . In a state of perfect solution. 

 2. In a state of beginning precipitation. Or, 3. Completely precipitated, and 

 falling in drops of rain. In the ist instance, where the water is in a state of 

 perfect solution, the air is clear, dry, heavy, and its powers of solution still 

 active, though it already contains a considerable proportion of water. In the 

 2d, the air becomes moist, foggy, its powers of solution are diminished, and it 

 becomes lighter in proportion as its water is deposited. It is a singular and 

 well-attested fact, that it never rains in the kingdom of Peru ; but that during 

 part of the year the atmosphere is constantly obscured with vapours, and the 

 whole country involved in what they call garuas, or thick fogs. 



It is not necessary to point out the causes which thus dispose the air to depo- 

 sit its dissolved water ; nor to consider with what bodies air has a stronger 

 affinity than with water ; neither to inquire how far the electrical fluid is 

 engaged in the process. It is sufficient to observe, that so long as these causes 

 have a general action on the air, they diminish its power of solution, and give a 

 damp and foggy state of the atmosphere ; that when they operate for a consider- 

 able proportion of the year, they produce a moist climate ; and that when they 

 more generally do not, and the air retains its moisture in a state of perfect 

 solution, the climate is dry. Consequently, that the moisture or dryness of a 

 climate, do not so much depend on the absolute quantity of water which is con- 

 tained in the air, as on the air being in a state of perfect or imperfect solution. 

 During long continued summer droughts, a very large proportion of water is 

 dissolved in the air ; notwithstanding this, the air is still dry, and continues to 

 be so as long as the water remains in a state of perfect solution ; but no sooner 

 are the powers of solution diminished, than what was before a dry, now becomes 

 a moist climate. ;.; 



In the 3d instance, the dissolved water may be either slowly precipitated and 

 fall in drizzling rain, or it may be more powerfully discharged in brisk rain ; or 

 there may be partial and sudden precipitations from particular regions, while 

 other parts of the atmosphere still retain their water in a state of perfect 



